It is a frequent requirement to superimpose one optical image on another. For example, in a pilot head-up display, aircraft instruments may be imaged and projected on to the helmet visor as a virtual image emanating from infinity, so as to be superimposed on the landscape image seen through the visor. As is well known, such a technique obviates the need for the pilot to remove his gaze from the landscape in order to read the instrument panel.
FIG. 1 is an optical ray diagram showing schematically a typical prior art optical combiner depicted generally as 10 for combining an image of an instrument 11 on to an external image seen by an observer 12. The instrument panels is imaged by an imaging objective 13 so as to project an incident beam of light 14 on to a beam splitter 15 having a partially reflective coating. A first portion 16 of the incident beam is reflected by the beam splitter 15 toward a concave reflector 17 which reflects the first portion 16 back through the beam splitter 15 as a second portion 18 towards the observer 12.
It is thus apparent that the beam splitter constitutes a major limitation in the overall efficiency of the optical system 10. This limitation derives from the fact that the beam splitter both reflects and transmits the incident beam and that any improvement to the reflectivity is at the expense of transmissivity and vice versa. For this reason optimal results are achieved when the reflectivity and transmissivity of the beam splitter are equal,
Thus, in an optimally configured system the beam splitter 15 may be assumed to reflect and transmit 50% of the light incident thereon. On this basis, the first portion 16 of the beam reflected by the beam splitter 15 has only 50% of the intensity of the original beam emanating from the instrument 11. The second portion 18 of the incident beam which finally reaches the observer 12 after yet again passing through the beam splitter 15 has a net intensity equal to only 25% of the intensity of the original light beam. This figure assumes 100% transmission through the imaging objective 13 and the concave reflector 17, which can never be met in practice. Thus, allowing for only 90% reflection by these two optics, the net intensity of the beam reaching the observer is close to 20% of the original beam. The net intensity can be increased somewhat to between 30 to 50% by employing polarizers or .lambda./4 plates for the concave reflector 17. However, any improvements to the transmissivity of the imaging objective 13 and to the reflectivity of the concave reflector 17 are marginal compared to the 25% net propagation of the beam through the beam splitter 15.
EP 0 303 742 discloses a head-up display for aircraft cockpits comprising an optical system for directing an image produced by a CRT on to a partly-reflective screen in the pilot's field of view. A prism having at least one curved face that acts as a lens replaces the fold mirror in previous systems allowing the CRT image to be directed towards the observer from a different direction than the external scene. The prism allows the optical system to be kept compact. In such a system, at least one combiner screen, or beam splitter, must still be used to present both images to the observer simultaneously.